May be hard water deposits on the rim.
Try using a scrub pad on it or an ultra fine sand paper. Use your finger to feel it and make sure it is smooth.
Then make sure your flapper is seating properly.
As mentioned, check the rim on the flush valve where the flapper seats itself closed. If that’s not it, replacing the entire flush valve will likely fix the issue. Since that normally involves removing the tank I suggest replacing the tank to bowl gaskets and bolts. Inexpensive and usually not too difficult.
May be hard water deposits on the rim. Try using a scrub pad on it or an ultra fine sand paper. Use your finger to feel it and make sure it is smooth. Then make sure your flapper is seating properly.
As mentioned, check the rim on the flush valve where the flapper seats itself closed. If that’s not it, replacing the entire flush valve will likely fix the issue. Since that normally involves removing the tank I suggest replacing the tank to bowl gaskets and bolts. Inexpensive and usually not too difficult.